Wireless telephony, e.g. mobile phone use, is a widely-used mode of communication today. Variable rate communication systems, such as Code Division Multiple Access (CDMA) spread spectrum systems, are among the most commonly deployed wireless technologies. Because of increasing demand and limited resources, a need arises to improve their capacity, fidelity, and performance.
Referring to prior art FIG. 1A, an illustration of multipath signal propagation between a conventional base station and a mobile phone is shown. A conventional base station 104 transmits a signal to a mobile station, e.g., phone, 102. Typically, the signal contains pilot information, that identifies the base station, and data information, such as voice content. A signal that can be transmitted directly to mobile phone 102 without interference, such as first signal 106a, provides the strongest signal. However, given the power limitations at which base station 104 can transmit the signal, and given the noise a signal may pick up, a need arises to improve the power and the SNR of the signal received at mobile phone.
Conventional methods will combine the portions of the transmitted signal that travel different paths to mobile unit 102. The multiple paths arise because of natural and man-made obstructions, such as building 108, hill 110, and surface 112, that deflect the original signal. Because of the paths over which these other signals travel, a time delay and performance deterioration intrinsically arises in the synchronization-sensitive and noise-sensitive data transmitted from base station 104 to mobile phone 102. However, to provide the strongest possible signal to a mobile phone, two or more of the signals from these multiple paths, e.g. path 106a-106d, may be combined. However, to efficiently combine and demodulate multipath signals, a need arises for a method to select the most worthwhile candidates from all the different multipaths received in mobile phone.
Corruption of a transmitted signal falls into two general categories: slowly-varying channel impairment and fast fading variation. Slowly-varying channel impairment arises from factors such as log-normal fading, or shadowing caused by movement or blocking as exemplified in prior art FIG. 1A, or slow fading. Slower variations, e.g., sub Hz, determine in effect, the “availability” of the channel. In contrast, only the fast fading variation affects the details of the received waveform structure and the interrelationships of errors within a message. Hence, a need arises for a method that effectively choose the properties of the signal that influence its condition for demodulation.
Referring now to prior art FIG. 1B, a graph of two conventional multipath signal strengths over time is shown. Graph 100b has an abscissa 122 of time and an ordinate of signal-to-noise ratio (SNR) 120, e.g. pilot EC/Io ratio. Third multipath signal 106c and fourth multipath signal 106d are shown as exemplary multipath signals received at mobile phone 102. Conventional methods typically select for combining, the multipath signals with the highest SNR. Thus, at time span A 124a, the dark line representing fourth multipath signal 106d has a higher SNR level than third multipath signal 106c, assuming both signals have the same noise level. However, at time span B 124b, the dashed line representing third multipath signal 106c has a higher SNR level. Given the closeness of the SNR, or of the signal to noise ratio, of these two multipath signals, the choice as to which signal will be chosen for the demodulator can oscillate back and forth.
This oscillation is a condition known as “thrashing.” The drawback with thrashing is that it consumes a significant amount of system resources, such as processor operations. During thrashing, the processor can be overloaded with operations that constantly assign and deassign the multiple demodulators to different multipath signals. Furthermore, thrashing may degrade the quality of the mobile phone 102 output signal, as the switching may cause a loss of data or an audible interference or it may introduce latency effects. Consequently, a need arises for a method to select the best multipath signal for combining while avoiding the effect of thrashing.
Furthermore, referring again to prior art FIG. 1A, conventional methods combine transmitted signals that travel different paths to mobile unit 102. The multiple paths arise because of natural and man-made obstructions, such as building 108, hill 110, and surface 112, that deflect the original signal. Because of the paths over which these other signals travel, a time delay and performance deterioration intrinsically arises in the synchronization-sensitive and noise-sensitive data that is transmitted from base station 104 to mobile unit 102. To provide the strongest possible signal to a mobile unit, two or more of the signals from these multiple paths, e.g. path 106a-106d, may be combined.
Corruption of a transmitted signal falls into two general categories: slowly-varying channel impairment and fast fading variation. Slowly-varying channel impairment arises from factors such as log-normal fading, or shadowing caused by movement or blocking from objects, as shown in prior art FIG. 1A, or from slow fading. Slower variations, e.g., sub Hz, determine in effect, the “availability” of the channel. In contrast, only the fast fading variation affects the details of the received waveform structure and the interrelationships of errors within a message. Interference on a signal can be caused by moving objects that temporarily block the signal, such as moving object 113 that interferes with signal 106b of prior art FIG. 1A. Based upon the characteristic differences of these signals, a need arises for a method of capturing a signal while avoiding the detrimental characteristics of fast fading or short fading variation encountered at the receiving unit.
Referring now to FIG. 1C, a flowchart of a conventional process used for implementing fingers in a communication device is shown. Flowchart 100c begins with step 1002. In step 1002, an inquiry determines whether an assigned signal fails to meet a threshold for combining. If an assigned signal does fail to the single threshold, then flowchart 100c ends. If the assigned signal satisfies the threshold, then flowchart 100c ends. In step 1004, the finger assignment is immediately deassigned, e.g. because it failed to meet the threshold. Following step 1004, flowchart proceeds to step 1006. In step 1006, the communication device waits for the searcher to assign a new finger.
Prior art FIG. 1C presents several problems associated with the conventional management of assigned fingers. The first problem deals with thrashing. The second problem deals with unnecessary latency. In step 1002, the only criteria by which fingers are deassigned is a single threshold for combining the signal. By using only a single threshold, third multipath signal 106c is immediately deassigned, per step 1004, as soon as it fails the threshold. Because of this limitation, one of the demodulating fingers must now wait for the searcher to identify a new multipath signal to be assigned, e.g., per step 1006. This latency occurs where third pilot 106c is deassigned and second multipath signal 106b is assigned.
In a different scenario, if no other multipath signals are available for demodulation, and a demodulating finger is available, then second multipath signal 106b may be constantly assigned and deassigned from the given demodulating finger based on its performance. That is, second multipath signal 106b frequently crosses the threshold value, thereby causing the communication device to frequently assign, deassign, and reassign a multipath signal to a demodulating finger that has no other worthy candidate multipath signals. This phenomenon of frequent assigning and deassigning is referred to as “thrashing.” Unfortunately, thrashing consumes a significant amount of system resources, such as CPU operations, by constantly performing tasks such as assigning and deassigning. Furthermore, thrashing may downgrade the quality of the output signal from the mobile unit 102. This is because the frequent changes in finger assignment, and its associated latency effects, may cause an perceptible degradation in the composite signal provided by the communication device to a user. Consequently, a need arises for a method of managing assigned fingers that avoids the problem of thrashing, and its associated side-effects.
Thus, an apparatus and a method are needed to improve the capacity, fidelity, and performance of digital communication. More specifically, a need arises for a method to improve the power and the SNR of the signal received at mobile phone. In particular, a need arises for a method to select the most worthwhile candidates from all the different multipaths received in mobile phone for a subsequent demodulation and combining operation. Additionally, a need arises for a method which meets the above needs and which selects the best multipath signal for combining while avoiding the effect of thrashing. A further need exists for a method which meets the above needs and which captures a signal while avoiding the detrimental characteristics of fast fading variation encountered at the receiving unit. Specifically, a need exists to prevent the problem of latency caused by frequent or unnecessary changes in finger assignment.